Liquid crystal display devices perform display by utilizing the change of the orientations of liquid crystal molecules which is caused by a voltage applied across the liquid crystal layer. The orientations of the liquid crystal molecules which occur in the absence of an applied voltage across the liquid crystal layer (pretilt directions) are conventionally regulated by alignment films. For example, in TN-mode liquid crystal display devices, the pretilt azimuths of the liquid crystal molecules are regulated by performing a rubbing process on horizontal alignment films. In this specification, the pretilt azimuth refers to a component of a vector that is indicative of the orientation of a liquid crystal molecule in the liquid crystal layer in the absence an applied voltage, the component being in a plane of the liquid crystal layer (in a plane of the substrate). The pretilt angle, which is an angle formed by the alignment film and the liquid crystal molecule, is determined depending primarily on a combination of the alignment film and the liquid crystal material. The pretilt direction is expressed by the pretilt azimuth and the pretilt angle. In TN-mode liquid crystal display devices, the pretilt azimuths regulated by a pair of alignment films which oppose each other via the liquid crystal layer are set perpendicular to each other. The pretilt angle is about 1° to 5°.
In recent years, as a technology for controlling the pretilt directions of the liquid crystal molecules, the Polymer Sustained Alignment Technology (hereinafter, referred to as “PSA technology”) has been developed (see Patent Documents 1, 2, and 3). The PSA technology is a technology of controlling the pretilt directions of the liquid crystal molecules by means of a polymer formed in the liquid crystal layer. The polymer is formed by irradiating, after assemblage of a liquid crystal cell, a small amount of polymerizable material (e.g., a photopolymerizable monomer) mixed in a liquid crystal material with active energy rays (e.g., ultraviolet light) while a predetermined voltage is applied across the liquid crystal layer. The orientations of the liquid crystal molecules maintained during the formation of the polymer are sustained (memorized) even after removal of the voltage (in the absence of an applied voltage). Thus, the PSA technology is advantageously capable of adjusting the pretilt azimuths and pretilt angles of the liquid crystal molecules by controlling, for example, an electric field generated in the liquid crystal layer. Also, the PSA technology does not require a rubbing process and is therefore suitable to formation of a vertical alignment type liquid crystal layer that has difficulty in regulating the pretilt directions by means of a rubbing process. The entire disclosures of Patent Documents 1, 2, and 3 are incorporated in this specification by reference.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-357830
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-307720
Patent Document 3: Japanese Laid-Open Patent Publication No. 2006-78968
Patent Document 4: Japanese Laid-Open Patent Publication No. 2006-169518
However, in the PSA technology, an insufficient UV dose leads to a problem that an unreacted portion of a photopolymerizable compound remains in the liquid crystal layer. The remaining photopolymerizable compound gradually reacts due to light or heat from a backlight during use of the liquid crystal display device, so that the orientations of the liquid crystal molecules which occur during the use may be sustained (memorized). If this phenomenon occurs, it may emerge in the form of image sticking. An image sticking phenomenon which is detrimental to general liquid crystal display devices is caused by a DC component remaining in the liquid crystal layer and is therefore called “DC image sticking”, while the above-described image sticking phenomenon which particularly occurs in connection with the PSA technology is herein referred to as “polymeric image sticking”.
The ultraviolet light can damage the liquid crystal material and the organic materials of the alignment films and other constituents and can therefore decrease the reliability. Thus, the UV dose cannot be inordinately increased. As the duration of the ultraviolet irradiation increases, the tact time accordingly increases. This leads to an increase in production cost.
Patent Document 3 discloses that the amount of a residual monomer is preferably 0.02 mass % or less. However, in example 6 (table 2) which uses a liquid crystal display panel, the amount of the residual monomer cannot be decreased to 0.017 mass % without ultraviolet irradiation of more than 40 J/cm2. Even when the UV dose is increased from 30 J/cm2 to 40 J/cm2 in the second ultraviolet irradiation step, the amount of the residual monomer only decreases from 0.02 mass % to 0.017 mass %. Thus, it is understood that, in consideration of reliability and production cost, the decrease in the amount of the residual monomer by means of increasing the UV dose has a limit.
The technology disclosed herein was conceived in order to solve the above problems. One of the objects of the technology disclosed herein is to provide a liquid crystal display device fabricated using the PSA technology, in which the amount of a polymerizable compound remaining in the liquid crystal layer is sufficiently decreased without increasing the UV dose as compared with the conventional process.
A liquid crystal display device of the technology disclosed herein includes: a liquid crystal layer containing a nematic liquid crystal material; a pair of electrodes opposing each other via the liquid crystal layer; a pair of alignment films respectively provided between the pair of electrodes and the liquid crystal layer; and an alignment sustaining layer formed of a photopolymerized material on each of surfaces of the pair of alignment films which are closer to the liquid crystal layer, the alignment sustaining layer being configured to regulate a pretilt azimuth of a liquid crystal molecule of the liquid crystal layer during the absence of an applied voltage across the liquid crystal layer, wherein the pretilt azimuth of the liquid crystal molecule of the liquid crystal layer is regulated by the alignment sustaining layer during the absence of an applied voltage across the liquid crystal layer, the nematic liquid crystal material contains a liquid crystal compound having a terphenyl ring system as an indispensable component, and the liquid crystal layer further contains part of a photopolymerizable compound which is a source material of the photopolymerized material, a content of the photopolymerizable compound relative to the nematic liquid crystal material being less than 0.015 mol%.
In one embodiment, a content of the liquid crystal compound having the terphenyl ring system in the nematic liquid crystal material is in a range of not less than 1 mol % and not more than 25 mol %.
In one embodiment, the photopolymerizable compound includes a diacrylate monomer which has a liquid crystal skeleton or a dimethacrylate monomer which has a liquid crystal skeleton.
In one embodiment, the pair of alignment sustaining layers include a particle of the photopolymerized material which has a particle diameter of 50 nm or less.
In one embodiment, the pair of alignment films are vertical alignment films, and the nematic liquid crystal material has negative dielectric anisotropy.
In a liquid crystal display device of the technology disclosed herein, a nematic liquid crystal material included in a liquid crystal layer contains a liquid crystal compound having a terphenyl ring system as an indispensable component. The liquid crystal compound having a terphenyl ring system has the function of increasing the efficiency of a polymerization reaction of a photopolymerizable compound in the liquid crystal layer. Therefore, in an end-product liquid crystal display device, the content of the photopolymerizable compound remaining in the liquid crystal layer relative to the nematic liquid crystal material can be less than 0.015 mol %. Further, the UV dose need not to be increased as compared with the conventional process. Thus, the liquid crystal display device of the technology disclosed herein is also advantageous in terms of reliability and production cost.